Typically, planar magnetic acoustic transducers use a flat, lightweight diaphragm suspended in a magnetic field, rather than a cone attached to a voice coil. The magnetic field is typically produced by a planar array of bar magnets, the bar magnets spaced apart regularly, but aligned parallel to each other, the poles of the bar magnets oriented to be perpendicular to the layer the magnets form. The diaphragm is suspended above the magnets, and substantial portions of the electrically conductive circuit pattern run parallel to individual bar magnets, as when current passes through these portions of the circuit, an induced magnetic field will react with the field produced by the magnets, causing the conductor, and the attached diaphragm, to be drawn to or away from the magnets.
However, there are drawbacks to the magnetic arrangement in the classic planar magnetic acoustic transducer design. In simple configurations, the magnetic field imposed by the array of bar magnets not only permeates the volume in which the diaphragm operates, but also imposes a like-intensity magnetic field on the opposite side of the array. In most applications this opposite side magnetic field is wasted. In a more sophisticated configuration, a stator is applied on the backside of the array, to contain and redirect the opposite side magnetic flux to bolster the field acting on the diaphragm. The added mass of the stator is a drawback to this configuration, as is increased impedance to the passage of sound due to the spaces between bar magnets being covered by the stator, even partially, as when the stator is perforated or consists of separated strips.
Prior art Halbach magnet arrays rely on a cyclical rotation in magnetic orientation from magnet to magnet, as shown in FIG. 1, wherein the magnetic axis of each magnet is 90-degrees further rotated than it's preceding neighbor. For example, in Halbach magnet array 100, shown in cross-section, each consecutive magnet from left-to-right has a magnetic axis that is 90-degrees further counterclockwise than its predecessor, as the axis of magnet 102 is 90-degrees more counterclockwise than magnet 101. The valued property that emerges from this arrangement in Halbach array 100, is that rather than having a symmetrical field evenly distributed on either side of the array, the field 103 on one side is greater than it would be for an traditional array of alternating magnets (which would correspond to magnet 101 and every alternate magnet to either side, with the intervening magnets, e.g., 102, removed). The field 104 on the other side is nearly canceled. As with a stator, in a planar magnetic acoustic transducer application, Halbach arrays suffer from the added magnets (e.g., 102) obstructing the passage of sound. While the property of an intensified field 103 on one side and substantially canceled field 104 on the opposite side makes a Halbach array more efficient in one way, the added weight corresponding to doubling the count of magnets and the obstruction to the passage of sound (even when some of the magnets are drilled through), reduce the effectiveness of Halbach arrays.
Additionally, Halbach arrays are difficult to assemble. The individual magnets are not in equilibrium when arranged as a Halbach array, and the array will collapse into a jumble if not glued or otherwise supported and braced. In some cases, particularly with individual magnets 101, 102 that are strong, manual assembly of the array can be fraught with pinched fingers and frequent starting over again.
A need exists for a magnetic array, having the property of an intensified field on one side and a nearly canceled field on the other, suitable for efficient use in planar magnetic and other acoustic transducers.